1. Field of the Invention
The present invention relates to an improved process and apparatus, including improved gas nozzle devices, for sensing and regulating the soldering and desoldering of electrical circuit components to a printed circuit board. More specifically, the present invention relates to improvements or modifications in gas nozzle devices used for the attachment or detachment of electrical circuit components having a land grid array of solder connections to a companion array of circuit connections printed onto the surface of the printed circuit board.
2. State of the Art
A variety of soldering/desoldering machines are known for the attachment and detachment of electrical circuit components from areas of crowded printed circuit boards, adjacent to other closely-spaced soldered components which are not to be disturbed. Reference is made to commonly-assigned U.S. Pat. No. 5,044,072 for its disclosure of such an apparatus, including vacuum gas nozzles for holding an electrical component relative to a PCB surface and for directing hot gas through slots therein downwardly against peripheral component leads and corresponding peripheral solder pads on the PCB. The hot gas melts the solder to permit the attachment or detachment, as desired, of the component relative to the PCB surface.
While such known processes and nozzles produce excellent results with conventional peripheral-lead components, they are not satisfactory for use with face-down or land grid array (LGA) components, including ball grid array (BGA) components, solder grid array (SGA) components, and column grid array (CGA) components. Such components comprise a grid array of spaced solder connections on the undersurface thereof, for electrical connection to a companion array of spaced circuit connections exposed at the upper surface of the printed circuit board. Conventional soldering/desoldering machines and conventional vacuum/gas nozzles are known which apply heated gas at a predetermined fixed temperature and flow rate through a nozzle device, and through a component enclosed thereby, to heat the component body and melt the solder grid array therebeneath. Such known systems have important disadvantages. For example, the applied hot gas does not have a controlled flow path and therefore it can flow from the nozzle against adjacent components on a PCB, causing melting and shorting thereof. Also, since the flow path of the hot gas, such as inert nitrogen gas, is not directed to the area of the solder grid array, the array is melted by indirect heating in the presence of ambient air and oxygen, whereas solder joints formed where an inert atmosphere displaces the ambient air are recognized to be superior.
However, the most important disadvantage of known systems is due to the fact that they are regulated to operate at predetermined fixed temperatures and flow rates, and do not incorporate any means for the automatic adjustment of gas flow rate and/or temperature under changing conditions of use where the pre-set temperature and/or gas flow rate are inadequate or are excessive. For example, at initial start-up the printed circuit board is relatively cold and higher flow rates and/or gas temperatures are required for the soldering operation. However if a plurality of adjacent components are being soldered/desoldered, the successive operations require less heating and may be ineffective unless the apparatus is adjusted each time to regulate the flow rate and/or temperature for the changing conditions of use. Even if this is done, the adjustments must be on the basis of trial by error.
The use of LGA components is growing in view of the advances in technology and the complexity of electrical circuit components which require a greater number of electrical connections which the peripheral area of the component body is not able to accommodate. LGA technology permits a large number of terminal connections to be printed in the form of a grid or array on the undersurface of the component, which connections are hidden beneath the body of the component being attached or detached. The exclusion of peripheral or perimeter leads, extending outwardly from the component, enables components to be mounted in closer proximity to each other, which saves vital PCB surface space or area to enable an increase in component population or a decrease in PCB size.
Copending, commonly-owned U.S. Ser. No. 08/125,118, filed Sep. 21, 1993, now U.S. Pat. No. 5,419,481, enables the uniform heating of the land grid solder array on the undersurface of an LGA component, at the interface with a PCB, by supporting the LGA component relative to the PCB surface, and directing a continuous hot inert gas flow through restricted gas orifices in a supporting nozzle device, under the component and through said land grid solder array, and venting it away therefrom, until all of the solder balls or columns of the array become melted in contact with the corresponding companion array of spaced circuit connections on the upper surface of the PCB. If attachment or soldering of the component to the board is being carried out, the gas flow is discontinued to permit the solder array to resolidify to the PCB contact array. If detachment or desoldering is being carried out, the LGA component is lifted away from the PCB while the solder array is molten. If a new LGA is to be inserted, the PCB melted contact array is prepared and/or new solder materials deposited for the new LGA component. The new LGA component, containing its land grid solder array, is then supported in registration thereon and heated as discussed above.
It is a critical feature of the process and apparatus of said copending application that the solder-melting hot gas is supplied to the underside or multi-contact side of the LGA component from one or more directions, and is withdrawn or permitted to exhaust from one or more opposed directions, whereby a continuous hot gas flow is maintained through all of the spaced solder land islands of the land grid array for a sufficient period of time to produce melting thereof, commonly between about 1/2 and 2 minutes.
According to the copending application, the flow rate of the hot gas through the spaced solder islands, and the dwell time of the hot gas beneath the LGA component, are controlled by flow meters and length of process time which depends on the type of the device and the printed circuit board characteristics and the temperature of the gas, and/or by Controlling the rate of withdrawal of the hot gas through the gas outlet orifices, in order to produce the necessary melting of all of the spaced solder islands while minimizing dwell time, overheating and solder flow.
Among the problems encountered in attempting to minimize dwell time of the component within the nozzle, during the soldering/desoldering operation, are the sizes of the different nozzles and of the components being attached thereby, the number and size or volume of the spaced solder islands and their composition and melting point, the temperature of the apparatus and/or of the nozzle and/or the PCB, i.e., whether the operation is an initial start-up or a sequential cycle, and other variables which affect the temperature, flow rate and dwell time necessary to optimize the soldering/desoldering operation, while minimizing the impact on surrounding components or assemblies.